Route Summarization Example 5

Route Summarization Example 5
Sometimes you just cannot easily summarize a set of routes to one summary address. If might be
more advantageous to summarize them into more than one summary address. In this example, we
will show you how to determine when you should use more than one summary address. The
addresses are 169.254.100.0 through 169.254.200.0 and 167.1.200.0 through 167.4.2.0. We’ll
break it down like we did before into binary:
Number 1: 169.254.100.0= 10101001.11111110.01100100.00000000
Number 2: 169.254.200.0= 10101001.11111110.11001000.00000000
Number 3: 167.1.200.0= 10100111.00000100.00000010.00000000
Number 4: 167.4.2.0= 10100111.00000001.11001000.00000000
If you wanted to try and summarize the above addresses into single aggregate address, it would
be 160.0.0.0/4, because only the first four bits match among all four addresses, which is a very
large address range. We will need to use two separate addresses, which are 169.254.0.0/16 and
167.0.0.0/13. Here’s the non-binary way:
1. The interesting octet is the third octet for the first range. There are (200 – 100) + 1 = 101
networks that need to be included in the summary.
2. The block size we should start with is 128.
3. (101 div 128) × 128 = 0. So, the value of the interesting octet in the summary address
might be 0.
4. This starting point with a block size of 128 will cover from the values 0 through 127, inclusive,
so we have a problem. Increasing the block size to 256 and confirming that our starting
point is still valid makes our summary address 169.254.0.0.
5. 256 – 256 = 0, so this is the value of the mask in the third octet, all others being 0 and/or
255. Our mask for our summary address is 255.255.0.0, or /16.
For the second range, the following is true:
1. The interesting octet is the second octet. There are (4 – 1) + 1 = 4 networks that need to be
included in the summary.
2. The block size we should start with is 4.
3. (1 div 4) × 4 = 0. So, the value of the interesting octet in the summary address might be 0.
4. This starting point with a block size of 4 will cover from the values 0 through 3, inclusive,
so we have a problem. Increasing the block size to 8 and confirming that our starting point
is still valid makes our summary address 167.0.0.0.
5. 256 – 8 = 248, so this is the value of the mask in the second octet, all others being 0 and/or
255. Our mask for our summary address is 255.248.0.0, or /13.